Steerable drilling systems commonly use a drill string with a drill pipe, a bottom hole assembly, and a drill bit. The bottom hole assembly includes a downhole mud motor powered by drilling fluid to rotate the drill bit and a bent housing to angle the drill bit off centerline. The bottom hole assembly is carried by the drill string, which extends to the earth's surface and provides the drilling fluid to the bottom hole assembly.
For drilling straight sections of the wellbore conventional rotary drilling techniques are typically used. The drill string is rotated from the rig at the surface, and the bottom hole assembly with its downhole mud motor and bent sub are rotated along with the drill string. To drill a curved section of the wellbore, however, the downhole mud motor is used to rotate the bit, and the off-axis bent housing directs the bit away from the axis of the wellbore to provide a slightly curved wellbore section, with the curve achieving the desired deviation or build angle. When drilling curved sections, the drill string is not rotated, but merely slides along the wellbore.
The direction of drilling, or the change in wellbore trajectory, is determined by the tool face angle of the drill bit. The tool face angle is determined by the direction in which the bent housing is oriented. The tool face can be adjusted from the earth's surface by turning the drill string. The operator attempts to maintain the proper tool face angle by applying torque or angle corrections to the drill string using a rotary table or top drive on the drilling rig.
It is a characteristic of directional drilling that a substantial length of the drill string may be in intimate contact with and supported by the wellbore wall, thereby creating a substantial amount of drag. Friction is exacerbated when the drill string is not rotating but is in slide drilling mode. Such drill string friction makes it difficult to apply appropriate weight on bit to achieve an optimal rate of penetration and promotes the stick-slip phenomenon. Additionally, the drill string friction may cause the axial force required to slide the drill string to be so great that the downhole mud motor may stall the instant the drill string breaks free. Moreover, when drill string angle corrections are applied at the surface in an attempt to correct the tool face angle, a substantial amount of the angular change may be absorbed by friction without changing the tool face angle, and stick-slip motion may cause the operator to overshoot the target tool face angle correction.
In some cases, drill string friction can be reduced by rotatively rocking the drill string back and forth between a first angle and a second angle or between opposite torque values. However, the rocking may not sufficiently reduce the friction. Also, the rocking may unintentionally change the tool face angle of the drilling motor, resulting in substantial back and forth wandering of the wellbore, increased wellbore tortuosity, and an increased risk of stuck pipe.
In other cases, a rotary steerable device can be used in place of a downhole mud motor and bent housing. A rotary steerable device applies a modulated off-axis biasing force to the bit in the desired direction in order to steer a directional well while the entire drill string is rotating. As a result, the desired tool face and bend angle may be maintained while minimizing drill string friction. When steering is not desired, the rotary steerable device is set to turn off the off-axis bias. Because there is no drill string sliding motion involved with the rotary steerable system, the traditional problems related to sliding, such as stick-slip and drag problems, are greatly reduced. However, rotary steerable devices may be complex and costly.